I. Field of the Invention
The present invention relates to a wall made of injection moulded thermoplastic material, in particular an inflatable air bag cover for vehicles, designed so as to rupture along a predefined line of weakness under the action of a force acting on one face of the wall.
II. Description of Related Art
An inflatable safety air bag for vehicles is normally contained, before use, in a space bounded by a cover made of injection moulded plastic material. When the air bag is inflated by a gas under the action of a collision, it exerts a pressure on the internal surface of the cover, thereby rupturing the latter and enabling the air bag to unfold in the passenger compartment of the vehicle. In order to obtain a clearly defined rupture of the cover, a line of weakness is normally provided in the form of a localised reduction in thickness.
Such a localised reduction in thickness involves a restriction of the movement of material during the fabrication of the cover by injection moulding, leading to defects in the appearance and to stresses in the material.